High amperage molded case circuit breakers are required to interrupt large values of current extended through a plurality of downstream breakers to respective loads. Problems of course arise from the blade and contact mass required to carry the large current values and from the necessity to control the high energy arcs created on current interruption. These problems are compounded in high amperage molded case breakers since the size of the apparatus, while large, is substantially reduced from that provided in iron clad or open frame breakers.
Arc plate stacks are also provided to assist movement of the arc from the vicinity of the contacts toward a vent opening in the breaker housing. The vent opening should be designed to provide some back pressure but not excessive back pressure during current interruption in order to prevent the arc from either restarting if it should be blown out too quickly, or to enable the arc to extinguish. The vent opening should therefore be subdivided into a number of passages spaced in correspondence with the movement of the arc. Providing such spaced passages in the molded wall of the breaker housing is difficult as this requires mold parts that move transverse to the mold forming the main housing cavity.